Microporous polyolefin membranes are useful as separators for primary and secondary batteries. Such batteries include lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, etc. When a microporous polyolefin membrane is used as a separator for a lithium ion battery, the membrane's properties significantly affect the battery's performance. In particular, the microporous membrane's permeability, mechanical properties, shutdown properties, meltdown properties, etc., generally affect productivity and safety.
It is desirable for a battery to have a relatively low shutdown temperature and a relatively high meltdown temperature, which generally results in improved battery safety properties, particularly for high-capacity batteries exposed to relatively high temperatures during storage and/or use. High separator permeability is desirable for high battery capacity. A separator with relatively high mechanical strength is desirable because improving separator strength can result in improved battery assembly and fabrication efficiency.
In general, microporous membranes containing only polyethylene have a relatively low meltdown temperature of about 150° C. and a shutdown temperature of about 140° C., while microporous membranes containing only polypropylene have relatively high shutdown temperatures of about 155° C. and a meltdown temperature of about 165 to about 170° C. In addition, microporous membranes have been proposed which comprise both polyethylene and polypropylene.
Japanese Patent Application Nos. JP1999-269289A and JP 2002-338730A disclose the optimization of the polymer used to produce the membrane in order to improve the membrane's properties. The references disclose using polyethylene producing with single site catalyst to improve the membrane's shutdown temperature. The membranes disclosed in the references have a relatively low shutdown temperature but also relatively poor air permeability.
Japanese Patent Application No. JP2003-231772A discloses a microporous membrane comprising polyethylene copolymer having Mv of from 100,000 to 5,000,000. The membrane has improved air permeability and strength, but the membrane's shutdown temperature is >137° C. Japanese Patent Application No. JP2005-225919A also discloses a microporous polyolefin membrane comprising polyethylene produced with metallocene catalyst to improve battery lifetime.
PCT Patent Application No. WO 07/52663 discloses a microporous membrane comprising a polyolefin composition having specified melting characteristics. The microporous polyolefin membrane has a relatively high air permeability but also a relatively high shutdown temperature.
While improvements have been made, membranes having relatively low shutdown temperature and relatively air permeability and high strength are desired.